This invention relates to vehicle mounted plows. More specifically, the invention relates to an improved plow lift device for attenuating shocks normally imparted to a vehicle as the vehicle moves over and across uneven terrain.
Known vehicles have snow plow blades affixed to a front end of the vehicle. These plow blades have a substantial mass and are very heavy. Moreover, the plow blades are releasably mounted to the frame of the vehicle and permit movement between a lowered working or operational position and a raised storage or suspended position.
A mounting structure or frame is attached to the forward end of the vehicle to hold the plow blade. Usually, the plow blade is hung or suspended forwardly of the mounting structure in a manner permitting vertical movement. A lift assembly operably positions the plow blade between its suspended transport or raised position and its operating or lowered position. Such lift assembly usually includes a lift arm adapted for movement about a pivotal axis. A driver controls movement of the lift arm and thereby the plow blade. A chain or cable system typically connects the plow blade to the lift arm.
As the vehicle is driven across uneven terrain or surfaces, i.e. railroad crossings, ruts, potholes and the like, the elevated plow blade can present significant problems and major difficulties. More specifically, when the plow blade is not in its lowered or work engaging position, forces are magnified by a moment arm around which the plow acts due to the mass of the plow blade being positioned on the far end of the plow frame away from the vehicle. The momentum imparted to the plow blade as the vehicle is driven over uneven terrain causes the suspended plow blade to bounce. That is, the uneven surface terrain causes initial upward movement of the plow blade toward the lift arm and subsequent forceful movement downwardly until the chains or cable limit its travel. Without any plow suspension system, when the plow blade reaches the travel limit of the associated chain or cable, such chain or cable will jerk the plow blade to a sudden stop, transmitting a sudden and sharp jolt of force back to the vehicle through the mounting structure. As will be appreciated, such bouncing of the plow blade happens repeatedly as the vehicle is driven or transported between locations.
The effect of the significant mass/weight of the plow blade on the vehicle suspension system is significantly magnified when considering the repetitive bouncing movement of the plow blade as the vehicle is driven from location to location. This repeated bouncing of the plow blade can adversely impact the vehicle""s suspension system by causing significant and rapid wear and tear thereof. Moreover, repeated bouncing of the plow blade can result in damage to a vehicle frame and/or the plow blade mounting structure. Furthermore, repeated bouncing of the plow blade causes extreme tensile stress loading of the chains or cables holding the plow blade in a suspended position. Of course, if such chains or cables should snap or break, the plow blade will crash thus enhancing the potential for accidents not only with the vehicle having the plow blade mounted thereon, but with other vehicles in the vicinity. Also, the potential bouncing of the plow blade can interfere with the steerability of the vehicle.
Furthermore, similar problems and difficulties may be encountered when the plow blade is lowered to its operating position. In this position, the plow blade is in contact with the road or off-road surface the vehicle is traveling on. To ensure proper contact between the plow and the surface to be plowed, the lift assembly is positioned such that the chains or cables do not support the full weight of the plow. The surfaces to be plowed, however, are commonly marred with uneven portions such as the joints associated with misaligned road surface segments, speed bumps, ruts, potholes and the like. When the plow blade contacts such imperfections in the road surface, the plow blade may be forced initially upward and then subsequently downward back to the road. Without any plow suspension system, the plow blade will freely plummet back to the ground or, if the surface imperfection is large enough, will snap to a sudden stop as the chains or cables are drawn taut. The resulting forces can be quite severe, and these forces are transmitted back to the vehicle through the mounting structure. As will be appreciated, such displacement of the plow blade may happen repeatedly as the vehicle operates to plow road and similar surfaces.
Thus, there is a need and a desire for a plow capable of attenuating shocks normally imparted to a vehicle by a plow blade as the vehicle moves over and across uneven terrain.
U.S. Pat. No. 4,947,563, issued to Paul T. Pfister, Jr., discloses a vehicle plow-suspension shock-absorber. Pfister involves a compression spring situated in line with the chain or cable that interconnects the plow blade to the lift arm. By adding such a compression spring, downward forces on the plow blade relative to the vehicle are dampened when the plow blade is in the raised storage or suspension position. Pfister, however, does not dampen the impact or shock to the vehicle caused by jerking motion of a lift arm initiated by instant release of weight from the upward movement of the plow blade, and has little if any effect when the plow blade is in the lowered working or operational position when the cable is slack.
In one alternative solution, U.S. Pat. No. 6,088,937 by the current inventors, discloses an improved plow with a suspension system attached to the chain or line that holds the plow blade. A two-way shock absorber is used and has a compression spring wrapped around a piston.
The present lift device offers another alternative solution addressing the problems recited above. Thus, it is a main object of the present invention to provide an improved plow lift device that provides a smooth ride in the plow vehicle despite vertical movement of the plow blade.
More specifically, it is an object of the present invention to provide an improved plow lift device that absorbs substantially all forces formed by the vertical movement of a plow blade, and prevents those forces from impacting the vehicle or suspension system of the vehicle holding the plow blade.
The invention may be broadly defined as a device for attenuating shock to a motor vehicle when a large load or object such as a plow blade is mounted thereon. The vehicle plow lift device attenuates the relative movement between the plow blade and vehicle, the resulting forces, and the transmission of such forces from the plow blade to the vehicle so that the forces are negligible or eliminated entirely when finally transmitted to the vehicle. Both upward and downward relative movement are dampened, and the invention operates both when the plow blade is in its raised storage or suspended position and when the plow blade is in its lowered working or operational position.
In one aspect of the invention, the lift device is.mounted on a vehicle and lowers, raises or carries a vertically movable assembly mounted to the vehicle. It has a primary lift arm in pivotal relation to the vehicle and a distal end disposed generally above the assembly. A secondary lift arm is pivotally mounted on the primary lift arm and has a free end. A hanging mechanism secures and suspends the assembly from the free end. A positioning mechanism raises and lowers one of the lift arms which ultimately raises and lowers the assembly or holds the assembly off of the ground on which the vehicle travels. An elastic member attenuates resultant forces originating at the assembly and transmitted from the secondary lift arm to the primary lift arm which prevents transmission of the forces to the vehicle.
Another aspect of the invention is that it comprises a compression spring that acts in unison with a dampener, such as a two-way shock absorber. In more detail, as the vehicle is driven between locations, the positioning means is normally conditioned to elevate the plow blade to a raised storage or suspended position. In such position, in accordance with the present invention, the compression spring and dampener of the lift device, through the secondary lift arm, resiliently suspends the plow blade in a raised position. When unstable road conditions are encountered, such as road divots, potholes, unstable railroad crossings, medians, and the like, the lift device controls movement of the raised plow blade to decrease or eliminate shock to the vehicle. When the plow blade moves upwardly in response to the vehicle moving over rough or bumpy terrain, the sudden release of weight initially causes the compression spring to expand and lift the secondary lift arm upward. However, the continued expansion of the spring and the compression of the piston of the two-way shock absorber eventually combine to slow or stop the upward movement of the secondary lift arm and absorb the upward forces.
When the plow blade falls back downward pulling the secondary lift arm downward as well, the two-way shock absorber receives a tensile force and is lengthened while the compression spring is compressed. Again, the combination of spring and dampener slows and controls the downward movement of the plow blade thus attenuating the shock imparted to the vehicle effectively creating a free xe2x80x9cfloatingxe2x80x9d weight. By floating the weight of the plow blade, the present invention significantly attenuates the shock imparted to the vehicle as compared to the dead weight of a plow blade merely suspended directly from a single lift arm secured to the vehicle. While either the compression spring or dampener could be used alone, the combination of the two creates a truer xe2x80x9cfloatingxe2x80x9d configuration where the plow blade bounces up and down without transmitting those forces to the vehicle.
When the plow blade is in the lowered working or operational position, as is customary, the weight of the plow blade is substantially supported by the surface to be plowed. This arrangement ensures that the plow blade will make adequate contact with the plowing surface such that snow and similar objects on the surface may be removed by the plow blade. Thus, the secondary lift arm supports little to none of the weight of the plow blade. However, as is common in the field, due to the unevenness or other imperfections in the plowing surface, forces may be imparted to the plow blade which cause relative motion between the plow blade and the vehicle, and resultant forces transmitted to the vehicle. The lift device of the present invention allows this necessary movement but, under appropriate circumstances, can temporarily xe2x80x9cfloatxe2x80x9d weight of the plow and thus reduce the impact or shock which result when the plow blade impacts the plowing surface.